Apex domains cannot have CNAME records.
Question and response
Question
dig emailsrv.net. NS
Answer
emailsrv.net. 5 NS ns-1139.awsdns-14.org.
emailsrv.net. 5 NS ns-349.awsdns-43.com.
emailsrv.net. 5 NS ns-630.awsdns-14.net.
emailsrv.net. 5 NS ns-1563.awsdns-03.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0